In an inverter circuit that drives a three-phase AC motor or the like, a semiconductor module including switching devices on upper and lower arms, a rectifying device, and the like, has been used. As the switching device, for example, an insulated gate bipolar transistor (IGBT) has been used, and as the rectifying device, for example, a flywheel diode (FWD) device has been used.
In the inverter circuit, by switching on and off the switching devices of the upper and lower arms, the direction of a current flowing through a plurality of coils provided in a motor is controlled to drive the motor. During this switching between on and off, a surge voltage is generated by a parasitic inductance generated by a wiring and the like. Especially, when a large current flows in the semiconductor module, each device provided in the semiconductor module may be broken by the surge voltage.
For dealing with this, there has been proposed a method for suppressing the surge voltage by installing a capacitor in the semiconductor module. However, a new wiring is required as the capacitor is installed, leading to formation of a new parasitic inductance by a wiring for connecting the capacitor and another wiring.
Each loop area of these wirings changes depending on the position of the capacitor, and hence, this parasitic inductance may increase depending on the position of the capacitor. In this case, the surge voltage increases due to the increase in the parasitic inductance, and thus cannot be reduced sufficiently.
Therefore, for example, in a semiconductor module described in Patent Document 1, semiconductor devices of upper and lower arms and an insulating member are disposed on a front surface of a middle-side plate made of metal, a conductive plate is disposed on the insulating member, and a capacitor is disposed on the conductive plate. A high-side plate is made of metal, and is disposed on the semiconductor device of the upper arm. A low-side plate is made of metal, and is disposed on the semiconductor device of the lower arm. The high-side plate and the low-side plate are connected by the conductive plate and the capacitor.
With such a configuration, a current direction flowing through the conductive plate and a current direction flowing through the middle-side plate are opposite to each other, so that a magnetic field generated by a current flowing through the conductive plate and a magnetic field generated by a current flowing through the middle-side plate cancel each other. Hence, it is possible to reduce an effective inductance and suppress the surge voltage.